This invention pertains to heat exchangers, particularly to very high efficiency crossflow heat exchangers.
Heat exchangers are used in a wide variety of industrial, commercial, aerospace, and residential settings. Just three of many examples are the radiator of an automobile, the condenser of an air conditioner, and numerous aerospace applications. There is a continuing need for heat exchangers having greater efficiency and lower cost.
The function of many types of heat exchangers is to transfer as much heat as possible from one fluid (usually a liquid) to another fluid (usually a gas) in as little space as possible, with as low a pressure drop (pumping loss) as possible. It would be desirable to reduce the size of the heat exchanger needed for a given rate of heat exchange, if there were a practical and feasible way to do so.
As structures shrink, i.e., as their surface area-to-volume ratio increases, thermal coupling between the structure and surrounding medium (gas or liquid) increases. The improved coupling is especially important for heat exchange between solid surfaces and gases, because thermal resistance at the gas-solid interface tends to dominate overall heat transfer.
However, in prior heat exchangers, as the diameter of the fluid channels has decreased, the pressure gradient for a given bulk velocity through those channels has increased dramatically, which has limited the reduction in size that has been possible in prior heat exchangers. Attaining a high heat transfer rate in prior heat exchangers has required that the mass flow rate (or volumetric flow rate) of the gas be high, regardless of the coupling between the gas and the channel walls. In prior micro heat exchangers, the channel length to hydraulic diameter ratio, L/DH, has typically been quite high (similar to the ratios for macroscale heat exchangers), which requires very large pressure drops.
M. Kleiner et al., xe2x80x9cHigh performance forced air cooling scheme employing microchannel heat exchangers,xe2x80x9d IEEE Trans. Components, Packaging, and Mfg Tech., Part A, vol. 18, pp. 795-804 (1995) discloses a heat exchanger using tubes to duct air to a heat sink containing microchannels that appeared to have relatively high L/DH ratios. In one example, an optimum channel width was said to be 482 xcexcm for a channel length of 5 cm, or an L/DH ratio of xcx9c50. See also FIG. 1 of the Kleiner et al. paper.
A. Tonkovich et al., xe2x80x9cThe catalytic partial oxidation of methane in a microchannel chemical reactor,xe2x80x9d Preprints from the Process Miniaturization: 2nd International Conference on Microreaction Technology, pp. 45-53 (New Orleans, March 1998) discloses a microchannel reactor formed of stacked planar sheets, used for non-equilibrium methane partial oxidation. The channels were described as having heights and widths between 100 xcexcm and 1000 xcexcm, and lengths of a few centimeters.
U.S. Pat. No. 4,516,632 discloses a microchannel crossflow fluid heat exchanger formed by stacking and bonding thin metal sheets (slotted and unslotted) on top of one another. Successive slotted sheets are rotated 90 degrees with respect to one another to form a crossflow configuration. The heat exchanger was said to be suitable for use in a Stirling engine having a liquid as the working fluid. The heat exchanger was required to be capable of accommodating liquids at variable pressures as high as several thousand pounds per square inch. As depicted, the channels appear to have relatively high L/DH ratios.
U.S. Pat. No. 5,681,661 discloses a heat sink formed by covering an article of manufacture, which may have macroscopic surfaces, with a plurality of HARMs, namely microposts. See also WO 97/29223. High aspect ratio microstructures (HARMs) are generally considered to be microstructures that are hundreds of micrometers in height, with widths usually measured in tens of micrometers, although the dimensions of particular HARMS may be greater or smaller than these typical measurements. HARMs may be made of polymers, ceramics, or metals using, for example, the three-step LIGA process (a German acronym for lithography, electroforming, and molding). There is no disclosure of any fluid-to-fluid heat exchanger.
D. Tuckerman, et al. xe2x80x9cHigh-performance heat sinking for VLSI,xe2x80x9d IEEE Electron. Device Letters, Vol. 2, No. 5, pp. 126-129 (May 1981) discloses the removal of heat from a silicon substrate using a water-cooled, microchannel heat sink at a pressure drop up to 31 psi.
R. Wegeng et al., xe2x80x9cDeveloping new miniature energy systems,xe2x80x9d Mechanical Engineering, pp. 82-85 (Sept. 1994) discloses a two-phase, vapor-compression refrigeration cycle, micro heat pump comprising compressors, condensers, and evaporators. The condensers and evaporators incorporated microchannels having cross-sectional dimensions on the order of 50 to 1000 microns. Using the refrigerant R-124 in such a heat pump, it was reported that in proof-of-principle tests an overall heating rate of 6 to 8 watts was achieved with an R- 124 flow of about 0.2 gram per second, a temperature difference of 13xc2x0 C., and a pressure drop of 1 psi.
The Internet page xe2x80x9cMicro Heat Exchangersxe2x80x9d (1998) depicts a miniaturized plate heat exchanger consisting of several layers of microstructured plates, intended for the countercurrent flow of fluids (presumably, liquids) in the different layers. In compliance with M.P.E.P. xc2xa7 608.01, the citation for the hyperlink to this Internet page has been deleted from the specification, but the citation should appear on the first page of the issued patent under the heading xe2x80x9cOther Publications.xe2x80x9d In addition, a printed copy of this reference is located in the file history of this patent.
Car radiators have a cross flow design that typically uses only the air that flows over the radiator""s coils by virtue of the pressure drop associated with the motion of the automobile. A commonly used measure of performance for a car radiator is the ratio of heat transfer: frontal area, divided by the difference between the inlet temperatures of the coolant (usually a water-ethylene glycol mixture) and of the air. For state-of-the-art innovative car radiators, this figure is typically about 0.31 W/K-cm2. However, these automobile radiators are quite thick (xcx9c2.5 cm or more). See, e.g., R. Webb et al., xe2x80x9cImproved thermal and mechanical design of copper/brass radiators,xe2x80x9d SAE Technical Paper Series, No. 900724 (1990); and M. Parrino, et al., xe2x80x9cA high efficiency mechanically assembled aluminum radiator with real flat tubes,xe2x80x9d SAE Technical Paper Series, No. 940495 (1994).
We have discovered an extremely high efficiency, crossflow, fluid-fluid, micro heat exchanger formed from high aspect ratio microstructures. To concurrently achieve the goals of high mass flow rate, low pressure drop, and high heat transfer rates, the novel heat exchanger comprises numerous parallel, but relatively short microchannels. The performance of these heat exchangers is superior to the performance of previously available heat exchangers, as measured by the heat exchange rate per unit volume or per unit mass. Typical gas channel lengths in the novel heat exchangers are from a few hundred micrometers to about 2000 micrometers, with typical channel widths from around 50 micrometers to a few hundred micrometers, although the dimensions in particular applications could be greater or smaller. The novel micro heat exchangers offer substantial advantages over conventional, larger heat exchangers in performance, weight, size, and cost.
The novel heat exchangers are especially useful for enhancing gas-side heat exchange. Some of the many possible applications for the new heat exchangers include aircraft heat exchange, air conditioning, portable cooling systems, and micro combustion chambers for fuel cells.
The use of microchannels in a cross-flow micro-heat exchanger decreases the thermal diffusion lengths substantially, allowing substantially greater heat transfer per unit volume or per unit mass than has been achieved with prior heat exchangers. The novel cross-flow micro-heat exchanger has performance characteristics that are superior to state-of-the-art innovative car radiator designs, as measured on a per-unit-volume or per-unit-mass basis, using pressure drops for both the air and the coolant that are comparable to those for reported innovative car radiator designs.
The crossflow of the two fluids is advantageous since the temperature of coolant approaches equilibrium over the distance of just a few channel diameters. In most prior micro heat exchanger designs, the fluids have flowed in the plane of the heat exchanger, through relatively long channels, which requires a substantially greater pressure drop than is required by the present invention. As the hydraulic diameter of a fluid channel decreases at a constant fluid velocity, the convection heat transfer coefficient increases, as does the surface area-to-volume ratio. For the fluid temperature to change by a given amount in otherwise identical systems, the required L/DH ratio decreases as the hydraulic diameter decreases. After the fluid approaches thermal equilibrium with the channel wall (which occurs over the distance of a few DH), no significant additional heat transfer occursxe2x80x94thereafter a longer L produces a greater pressure drop but is of little benefit to heat transfer.
The invention allows the inexpensive manufacture of high-efficiency heat exchangers capable of supporting high heat fluxes, and high ratios of heat transfer per unit volume (or per unit mass), with minimal entropy production (i.e., a minimal combination of pressure drop and temperature difference between the two fluids exchanging heat). Thermal resistance at the gas/heat exchanger surface boundary is dramatically reduced compared with prior designs.
The dimension of the heat exchanger across which the first fluid flows is less than about 6 mm, preferably less than about 2 mm, most preferably less than about 1 mm. By contrast, it is believed that no prior gas-fluid cross-flow heat exchangers have been thinner than about 2 cm in the direction of the first fluid flow.
The dimension of the coolant fluid channel, measured perpendicular to the direction of the coolant fluid flow and measured perpendicular to the direction of the first fluid flow, is less than about 2 mm, preferably less than about 500 xcexcm.
The density of the gas channels is at least about 50 per square centimeter, preferably at least about 200 per square centimeter, and in some cases as much as about 1000 per square centimeter or even greater.